score = input("请输入学生分数：")
if not score.isdigit():
    print("输入的不是数字")

else:
    score = int(score)
    if 90 <= score <= 100:
        print("A")
    elif 80 <= score < 90:
        print("B")
    elif 60 <= score < 80:
        print("C")
    elif score < 60:
        print("D")
